CHEMICAL ECOLOGY, ECOSYSTEM TIPPING POINTS, AND THE DEATH SPIRAL
OF CORAL REEFS
Mark Hay. School of Biology, Georgia Institute of Technology, Atlanta, GA 30332
Coral reefs are in precipitous global decline. In the last 3-4 decades, coral cover has
declined by 80% throughout the Caribbean and 50% throughout the tropical Pacific,
with seaweeds commonly replacing corals. Much of the decline and lack of recovery
can be attributed to alterations in fundamental biotic interactions that are mediated via
bioactive secondary metabolites. Experiments on Fijian reefs demonstrate that
herbivory by specific mixes of herbivorous fishes is critical for suppressing chemicallyrich seaweeds that damage corals on contact via allelopathic lipids on their surfaces. Of
equal importance is how coral and fish larvae respond to chemical cues from overfished
areas dominated by seaweeds versus no-take marine protected areas (MPAs)
dominated by corals. Recruiting fishes and corals chemically sense and are attracted to
coral dominated areas protected from fishing while being chemically repulsed by
seaweed dominated areas that are overfished. Attraction and repulsion are cued by
odors from specific corals and seaweeds that best predict reef quality. Both recruiting
fishes and coral larvae refused to settle in overfished, seaweed dominated areas, but
recruited readily to immediately adjacent reefs where fishing was banned and corals
dominated. These chemically-cued behaviors can close the open nature of marine
populations, suppress larval export from coral dominated marine protected areas to
degraded reefs, and prevent recovery of coral and fish populations once reefs degrade
and become dominated by seaweeds.